Solder joint failure analysis
01/01/2003
Dye penetrant technique
BY TERRY BURNETTE and THOMAS KOSCHMIEDER
Solder joint integrity is of paramount importance for the reliability of an electronic package attached to a printed circuit board (PCB). Inspection of the solder joints once attached to a PCB can be virtually impossible for ball grid array (BGA) packages. Using hot-air rework stations to remove the package destroys all evidence of failed solder joints. Cross-sectioning a BGA that is mounted to a PCB to identify the failed solder joint is time consuming. In contrast, a method called "dye-n-pry" can be completed in a relatively short period of time, allowing one to understand whether a PCB module failure is due to a solder joint failure or other problems.
The Motivation
The electronics industry has moved rapidly to a variety of BGA packages, including ceramic BGAs (CBGA), plastic BGAs (PBGA) and mold-array process BGAs. The packages are made by different companies and are produced by a wide variety of processes, yet they all have balls connecting the BGA package to the PCB.
Typically, the BGA packages have multiple rows of balls. One can visually inspect the outer ball row (perimeter) for failed solder joints, but it is difficult to visually inspect rows of balls farther under the package. X-ray inspection can only provide certain clues as to the root cause of an open solder joint, and it has limited capabilities for finding failures related to solder joint fractures and open interconnects. This problem of trying to identify a failed solder joint under the package is important to the entire industry.
The dye-n-pry method allows for the failed solder joint to be marked with a dye regardless of the location in the BGA. Additionally, one can visually inspect the package after the pry operation to identify the failed solder joint and determine the cause for the failure. The method has limited needs for equipment and can be performed almost anywhere.
 Figure 1. Dye is applied along the package edge so that it can seep into defective solder joints. |
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Dye-N-Pry Basics
To successfully complete the dye-n-pry process, a few pieces of equipment are needed. The first is dye, essentially a colored solvent-based system, which has the ability to creep into very fine solder joint cracks. Additionally, the dye must dry relatively quickly in any readily available oven. For dye, one can use layout fluid, a material used in machine shops for marking materials like steel. Layout fluid dye nominally cures at ambient temperature but can be cured quickly by baking at 125°C for at least 15 minutes.
To apply the dye to the package, a pipette, syringe or similar delivery technique places a bead of the dye along the package edge (Figure 1). In Figure 1, the red arrow shows the direction along which the pipette dispenses the dye. Sufficient dye must be provided to completely fill the underside of the BGA and encase all the solder interconnects. When the dye starts coming out from under the BGA package edges opposite the dispense side, enough dye has been applied. Tilting the PCB a bit helps the dye flow under the package.
After completing the dye application, there is an optional step that can help in getting the dye into very fine cracks. Placing the PCB into a vacuum chamber to remove the air helps the dye get into fine cracks that otherwise would be blocked by the trapped air pockets. Note that some of the solvent from the dye also will be removed in the vacuum chamber. A strong vacuum pressure is not important for this process.
The PCB then is placed into an oven to cure the dye in place. Cure times vary, but a minimum of 15 minutes at 125°C appears to work well with most standard layout fluids. The cure drives off any remaining dye solvent and hardens the dye in place.
Once the dye has cured, the BGA is ready to be removed from the PCB. There are numerous techniques available for BGA removal depending on the PCB design. The goal in removing the BGA is to fully break all of the good solder joints that exist between the BGA and PCB, thereby exposing the open joint that will be evident by the colored dye that has filled the crack or gap of the open connection. The good solder connections that were broken after dye curing will show either silver/gray solder or copper solder pads torn completely out of the BGA and PCB.
It should be understood that dye penetrant analysis is destructive to both the BGA and the PCB it was attached to. One method for removing the package is to pry it off with a screwdriver blade, although it is important not to jam the screwdriver blade into the BGA solder joints and cause additional damage to them. "Walking" around the package edge with the screwdriver blade rotating/lifting the package edge generally is successful. With time, the amount of rotation/lifting possible for the package increases as the solder joints begin to break.
With PCBs that are 0.062" (1.5 mm) or thinner, flexing the board in the X and Y directions typically will break the good solder connections, allowing the BGA to be easily removed with a flat head screwdriver. In cases where the PCB is thicker than 0.062" (1.5 mm) or the PCB is rigidized, flexing the board to break the good joints will not be an option. In these cases, it is necessary to remove the black plastic mold cap from the BGA before trying to separate the BGA substrate and solder balls from the PCB.
To remove the black BGA mold cap, the sharp edge of a putty knife or thin chisel should be inserted at the edge of the interface between the mold cap and the BGA, and then it can be struck with a hammer to wedge the sharp edge between the mold cap and substrate. Slight prying with the knife/chisel now will remove the mold cap. Once the mold cap is removed, pulling upwards with a pair of needle nose pliers can peel off the BGA substrate. The solder connections then will be fully visible.
 Figure 2. Black pad on PCB that uses electroless Ni/Au solder pad finish. |
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Analysis
Understanding the root cause of the various types of failed solder interconnects requires some level of experience at solder joint analysis. Figures 2 through 5 are post dye-n-pry pictures with descriptions of the root cause for each failure. All come from real field returns and are from different applications.
Figure 2 shows a problem that can occur for electroless Ni/Au finished solderable surfaces. Depending on various factors, a brittle intermetallic can form on the board surface at the solder joint. This situation generally results in a clean break along the intermetallic interface. When the dye is applied to the package solder joints in question, it will creep into the cracks. Once the package has been removed from the PCB, the broken intermetallic bond can be spotted easily because it will be covered with the dye.
 Figure 3. Expected CBGA fatigue-induced solder joint failure. |
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Figure 3 shows a classic example of solder joint fatigue cracks for a CBGA package temperature cycled until failure. The figure has a corner area of the CBGA solder joints visible looking at the PCB. The silver/gray areas in the red dye-covered round solder joints are the remaining mechanical connect between the package and solder joint. Several solder joints have no mechanical contact left. CBGAs fail by distance from neutral point driven coefficient of thermal expansion (CTE) mismatch. That leads to the shown behavior of the remaining connect areas "rotating" as one goes around the CBGA package corner. The connect areas are perpendicular to a line connecting the center of the package with any particular solder joint and, thus, the connect areas "rotate" around the package.
 Figure 4. Open solder joint due to dry flux, and lifted Cu solder pads due to PCB bending. |
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Cold solder joints (Figure 4) also can be found with dye-n-pry. In this case, the reflow profile for the board assembly had not been optimized for the flux in the solder paste. The soak zone was too long, causing the flux solvents to dry out before reflow occurred. This dried out flux can form a barrier between the package solder ball and the solder paste on the PCB. During reflow, both the package solder ball and the PCB solder paste melt, but because of the dried flux barrier, they never join. Such boards can pass electrical test but eventually will fail for opens due to normal board bending. In fact, the right side of the figure shows cracks in the solder joint from bending the board during electrical test to see if it could be made to work.
Intentional or unintentional board bending can be a significant cause of cracked solder joints. Figure 5 shows a case where boards in a panel were separated improperly, causing the board to bend and leading to cracked solder joints. The dye creeped into the broken solder joints and was clearly visible after package removal.
 Figure 5. Fractured solder joints at PCB solder pads due to PCB bending. |
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Conclusion
The failure analysis technique of dye-n-pry for solder joints has been presented and described. The method provides quick, effective, inexpensive and locally available techniques for finding and documenting solder joint cracks.
Illustration by Gregor Bernard
Terry Burnette and Thomas Koschmieder may be contacted at Motorola SPS, 3501 Ed Bluestein Blvd., MS:F25, Austin, TX 78721; (512) 933-5783; E-mail: [email protected]; [email protected].